Mainly a swelling mat—a mixture of ceramic fibers, vermiculite micas and organic binders—is currently used to hold and insulate ceramic monoliths in motor vehicle exhaust gas units. The felt mat is wrapped around the monoliths and is pressed in height and thickness by insertion and closing the housing (or by pushing into a pipe or wrapping around with an open pipe and tensioning and closing the pipe). As a result, the swollen mat builds up a compressive strength against the monolith and the housing and holds the ceramic monoliths in the exhaust gas unit by friction between the monolith and the swelling mat, on the one hand, and between the housing and the swelling mat, on the other hand, under load exerted by the forces occurring during the operation (pressure loss on the monolith, acceleration forces on the system). When the temperature rises during the operation, the holding forces of the swelling mat increase due to the thermal tensioning of the vermiculite against the ceramic fibers. The pressing of the swelling mat increases with rising temperature and also with increasing friction. This functions very well in the case of round catalytic converters with uniform, circular gap. The swelling mat becomes adjusted with rising temperature considerably more than what the system loses in tension due to the housing pipe widening against the monolith due to thermal expansion. However, geometrically more unfavorable shapes, such as triangles, polygons and flat ovals, so-called racetracks, are also used besides round monoliths to utilize the free cross sections in the tunnel of a vehicle bottom for the largest possible cross-sectional areas of the monolith (to minimize the pressure loss).
In the case of such cross-sectional shapes, the rigidity of the housing is usually not sufficient during assembly or even during the operation to maintain a constant mounting gap for the swelling mat. Expansion by elasticity takes place in the larger radii or in the flatter areas of the housings during assembly and widening additionally takes place during the operation due to the increased pressing of the swelling mat under elevated temperature. This leads to a nonuniform pressure distribution on the circumference. The highest pressures and consequently the strongest holding forces are generated in the small radii of the cross section and the gaps increase at the large radii. However, the erosion resistance of the swelling mat decreases with increasing gaps. It becomes susceptible to gas pulsations penetrating into the swelling mat and to vibrations. The mica grains practically become detached from the composite and break apart the fibers located next to them in the case of small swelling mat thicknesses and very high loads (accelerations, pulsations, temperatures, and rates of temperature change). They create small cavities for themselves in the mat, which become increasingly larger during the further operation and finally lead to the emptying of the mat, to the formation of a nonpurified exhaust gas bypass flow around the monolith and ultimately to the separation of the monolith with complete failure of the system.
To take the above-mentioned problem fundamentally into account, it is proposed according to DE 296 11 788 U1 that more erosion-resistant mat inserts, namely, Saffil inserts, be used in the larger radii of oval housings and monoliths in the case of a composite mat, while the aforementioned swelling mat material can be maintained in the smaller radii as before. To save expensive Saffil in less critical areas, each Saffil insert has recesses or grooves, which extend flush end to end with corresponding projections or tongues of swelling mat sections, in the axial center of the patchwork mat. These individual mat sections are held together by an adhesive tape. The drawback is the sharp-edged, rectangular cut of the tongue-and groove connection at the joints, which continues to be associated with problems in terms of erosion at the projecting corners of the swelling mat and the tongues. Furthermore, difficulties arise in connection with handling and assembly, namely, the problem of projecting corners being caught and of these corners being folded over during the mounting in the half shell or in a tubular housing. Another drawback is the fact that a large amount of waste of the expensive Saffil inserts is generated when the inserts are cut out of a basic mat of the corresponding material. This also applies to the waste generated during the cutting out of the swelling mat sections.
The applicant's own patents DE 38 35 841 (“Soft Intermediate Ring at the End of or Between the Monoliths,” EP 0 387 422 (“Ceramic Ring”) and EP 0 472 009 (“Wire Mesh Between the Monoliths”) shall be referred to concerning other prior art. Both interrupted swelling mats in mounts with a plurality of monoliths and setback swelling mats with edge protection arranged in front of them are described in these patents. The edge protection also consists of fibrous material with sealing function. The use of other materials at the transition between the monoliths in the swelling mat mount in the form of an inner swelling mat protection or of a completely different elastic part is also described there.